Method and apparatus to counterbalance intrinsic positive end expiratory pressure

ABSTRACT

The invention prevents dynamic airway compression during ventilatory support of a patient. The respiratory airflow is determined by measurement or calculation, and a measure of the degree of dynamic airway compression is derived from the determined airflow. This measure is servo-controlled to be zero by increasing expiratory pressure if the measure of the degree of dynamic airway compression is large or increasing, and by reducing expiratory pressure if the measure of the degree of dynamic airway compression is small or zero.

This application is a continuation of U.S. application Ser. No.10/915,080 filed Aug. 10, 2004, now U.S. Pat. No. 6,915,803, which is acontinuation of U.S. application Ser. No. 10/461,776 filed Jun. 12, 2003now U.S. 6,854,462 which is a continuation of U.S. application Ser. No.09/482,251 filed Jan. 13, 2006 now 6,588,422 which claims the benefit ofUnited States Provisional Application No. 60/116,121, filed Jan. 15,1999.

FIELD OF THE INVENTION

This invention pertains to the field of ventilatory support forrespiratory failure, particularly due to lung disease, and in particularto automatically providing sufficient end expiratory pressure to unloadintrinsic positive end expiratory pressure (PEEPi).

BACKGROUND OF THE INVENTION

Subjects with chronic airflow limitation (CAL) due, for example, toemphysema and chronic bronchitis, may require ventilatory assistance,particularly during periods of acute exacerbation, or routinely atnight.

Ventilatory support can reduce the work of breathing, reduce thesensation of breathlessness, and improve blood gases (oxygen and carbondioxide levels). In subjects with CAL, most of the work of breathing isdue to the high airway resistance. Approximately two thirds of thisresistance is relatively fixed, and due to narrowing of the airways.However, of the order of one third of the resistance is due to dynamicairway compression during expiration. Dynamic airway compression occurswhen the pleural pressure exceeds the pressure in the lumen of theairway during expiration, causing flow to become independent of effort.

In a normal subject, the alveolar pressure decays exponentially duringexpiration, so that the expiratory flow and alveolar pressure (relativeto atmospheric) are both approximately zero at the end of expiration,and the lungs and chest wall have returned to their passive equilibriumvolume V_(R). In patients with CAL, however, as a result of dynamicairway compression and fixed reduced expiratory flow rate, it is notpossible for the lungs to return to V_(R) in the time allowed before thestart of the next inspiration. The chest is hyperinflated. The alveolarpressure remains positive, on the order of 5 to 15 cmH₂O at the end ofexpiration. This raised alveolar pressure is termed intrinsic positiveend expiratory pressure, or PEEPi. (Other names for the phenomenon arecovert PEEP and occult PEEP.)

An important effect of the hyperinflation is that the patient mustovercome the elastic recoil of the hyperinflated chest wall beforeinspiratory airflow can commence. The PEEPi is said to act as aninspiratory threshold load. A further undesirable effect of PEEPi isthat during artificial mechanical ventilatory support, it interferessubstantially with the triggering of the ventilator, causingpatient-machine asynchrony.

It is now well understood that the addition of a counterbalancingexternal positive end expiratory pressure (called external PEEP, or justPEEP), approximately equal in magnitude to PEEPi, is of great benefit.First, it prevents dynamic airway compression, permitting greaterexpiratory airflow. Second, it balances the inspiratory threshold load.Third, it improves triggering of a ventilator by the patient.

Use of excessive PEEP, however, can be disadvantageous and evendangerous. Excessive PEEP above and beyond PEEPi will cause yet furtherhyperinflation. This will result in stiffening of the lung and chestwall, and an increase in the elastic work of breathing. It will alsocause reduced cardiac output, and can lead to barotrauma. Further, thepeak inspiratory airway pressure during ventilatory support cannot bearbitrarily increased without either exceeding the capacity of theventilator, or reaching a pressure that is itself dangerous. Finally,excessive external PEEP will also reduce the possible airway pressureexcursion or headroom available for lung inflation.

Therefore, it is advisable when applying external PEEP to set theexternal PEEP as close as possible to PEEPi. Since PEEPi varies fromtime to time, depending on a number of factors including, for example,the resistance of the small airways and the respiratory rate, both ofwhich change with changing sleep stage, chest infection, orbronchospasm, it is desirable to be able to make multiple, or evencontinuous, measurements of PEEPi in order to optimize external PEEP.

A typical patient in an intensive care unit is heavily sedated andparalyzed during ventilatory support, and it is straightforward tomeasure the PEEPi. It is necessary only to occlude the airway duringlate expiration, and measure the airway pressure, which, after a fewseconds of equilibration, will equal static PEEPi. Since the lung injuryin CAL is usually markedly heterogeneous, different alveoli will havedifferent end expiratory pressures, and static PEEPi is therefore aweighted average across all alveoli.

Another known method which is suitable for use in the paralyzed sedatedpatient is to measure the airway pressure at the start of machineinspiratory effort, and again at the start of actual inspiratoryairflow. The difference between these two pressures is the dynamicPEEPi. Dynamic PEEPi reflects the end expiratory pressure in the leastabnormal lung units, and substantially underestimates static PEEPi.

These simple methods do not work for patients who are not sedated andparalyzed, and who are making spontaneous breathing efforts, becausethey do not take into account the patients' own respiratory muscleefforts.

One known method that is used with such patients requires a Mullermanoeuvre (maximal inspiratory effort) during catheterization of theoesophagus and stomach, and is therefore completely unsatisfactory forrepeated or continuous measurements in the ambulatory patient or thepatient who is being treated at home long-term.

Methods for measuring the airway conductance in spontaneously breathingpatients using oscillometry are taught by Peslin et al., RespiratoryMechanics Studied by Forced Oscillations During Mechanical Ventilation,Eur Respir J 1993; 6:772-784, and by Farre et al., Servo ControlledGenerator to Measure Respiratory Impedance from 0.25 to 26 Hz inVentilated Patients at Different PEEP Levels, Eur Respir J 1995;8:1222-1227. These references contemplate separate measurements forinspiration and expiration. Oscillometry requires modulation of theairway pressure at a high frequency, such as 4 Hz, and measurement ofthe resultant modulation of the respiratory airflow at that frequency.However, these references fail to describe servo-controlling ofventilation to increase or decrease PEEP so that the inspiratory aridexpiratory conductances are approximately equal.

Oscillometry has been used to control nasal CPAP (see U.S. Pat. No.5,617,846) or bilevel CPAP for the treatment of obstructive sleep apnea(see U.S. Pat. No. 5,458,137). The problem there is essentially oppositeto the problem under consideration here. In obstructive sleep apnea,there is increased resistance during inspiration, and the above twopatents teach that increased resistance during inspiration can betreated by an increase in pressure. In patients with CAL and dynamicairway compression, there is increased resistance during expiration.

There is no known method or apparatus which can automatically orcontinuously control a ventilator or CPAP apparatus in consciousspontaneously breathing patients in order to prevent expiratory airflowlimitation or to unload PEEPi in CAL.

Yet another known method for estimating PEEPi, taught, for example, byRossi et al., The Role of PEEP in Patients with Chronic ObstructivePulmonary Disease during Assisted Ventilation, Eur Respir J 1990;3:818-822, is to examine the shape of the expiratory flow-volume curve,which has been observed to be exponential, if there is no dynamic airwaycompression. The reference further notes that in the absence of PEEPi,the flow-volume curve becomes a straight line.

The above known art only contemplates the application of an externalpressure which is constant during any one expiratory cycle. However, theelastic recoil of the lung is higher at high lung volume, and lower atlow lung volume. Therefore, it may be advantageous to find the minimumexternal pressure at each moment in time during an expiration that willprevent dynamic airway compression during that expiration.

It is an object of our invention to vary the ventilatory pressure duringexpiration as a function of the degree of the patient's dynamic airwaycompression.

It is another object of our invention to vary the ventilatory pressureautomatically based solely on continuous measurements that are alreadytaken in conventional CPAP and ventilator apparatuses.

SUMMARY OF THE INVENTION

The present invention seeks to provide continuous and automaticadjustment of the expiratory pressure during ventilatory support, so asto substantially prevent dynamic airway compression and unload intrinsicPEEP with the smallest amount of external expiratory pressure.

The basic method of the invention prevents dynamic airway compressionduring ventilatory support using a conventional interface to a patient'sairway such as a face mask, nose mask, or endotracheal or tracheotomytube, and providing the interface with an exhaust and a supply ofbreathable gas at a variable pressure as is known in the CPAP andventilatory arts. The respiratory airflow is determined by measurementor calculation, and a measure of the degree of dynamic airwaycompression is derived. This measure is servo-controlled, preferably tobe zero, by increasing expiratory pressure if the measure of the degreeof dynamic airway compression is large or increasing, and by reducingexpiratory pressure if the measure of the degree of dynamic airwaycompression is small or zero.

The measure of the degree of dynamic airway compression may be aninstantaneous or pointwise measure within any given breath, and the stepof servo-controlling the measure to be zero may similarly be performedpointwise within a given breath, so that the expiratory pressure issimilarly varied pointwise within a breath. As an alternative to thusbasing the airway compression determination and the servo control onmultiple airflow determinations made within each individual respiratorycycle, the derivation of the measure of the degree of dynamic airwaycompression and the servo-controlling of the airway compression may beperformed across a plurality of respiratory cycles.

During expiration, the expiratory pressure increase may be linear as afunction of expired volume as will be described below.

The measure of the degree of dynamic airway compression is preferablyderived by measuring the airway conductance separately during theinspiratory and expiratory portions of one or more respiratory cycles,and calculating the measure of the degree of dynamic airway conductanceas a function of the inspiratory conductance minus the expiratoryconductance, or alternatively as the ratio of the inspiratoryconductance to the expiratory conductance. The two separate conductancesduring inspiration and expiration may be measured by superimposing ahigh-frequency oscillation on the patient interface pressure, at a knownor measured amplitude, identifying the inspiratory and expiratoryportions of each respiratory cycle, measuring the component of therespiratory airflow at the high frequency separately over theinspiratory and expiratory portions of one or more respiratory cycles,and from these measurements and the determined pressure amplitudecalculating the inspiratory airway conductance and the expiratory airwayconductance.

Alternatively, the measure of the degree of dynamic compression may bederived from the shape of the expiratory airflow versus time curve. Themeasure is zero when the expiratory flow decays exponentially from themoment of the peak expiratory flow to end expiration, but is large whenthe expiratory flow decreases suddenly from the peak expiratory flow andis then steady but non-zero for the remainder of expiration. The measuremay be the ratio of the mean expiratory flow during approximately thelast 25% of expiratory time to the peak expiratory flow.

Further objects, features and advantages of the invention will becomeapparent upon consideration of the following detailed description inconjunction with the drawing which depicts illustrative apparatus forimplementing the method of our invention.

DETAILED DESCRIPTION OF THE INVENTION

In the drawing, a blower 1 supplies breathable gas to a mask 2 incommunication with a patient's airway via a delivery tube 3 andexhausted via an exhaust 4. Airflow at the mask 2 is measured using apneumotachograph 5 and a differential pressure transducer 6. The maskflow signal from the transducer 6 is then sampled by a microprocessor 7.Mask pressure is measured at the port 8 using a pressure transducer 9.The pressure signal from the transducer 6 is then sampled by themicroprocessor 7. The microprocessor 7 sends an instantaneous maskpressure request (i.e., desired) signal to a servo 10, which comparesthe pressure request signal with the actual pressure signal from thetransducer 9 to control a fan motor 11. Microprocessor settings can beadjusted via a serial port 12.

It is to be understood that the mask could equally be replaced with atracheotomy tube, endotracheal tube, nasal pillows, or other means ofmaking a sealed connection between the air delivery means and thesubject's airway.

The invention involves the steps performed by the microprocessor todetermine the desired mask pressure. The microprocessor accepts the maskairflow and pressure signals, and from these signals determines theinstantaneous flow through any leak between the mask and patient, by anyconvenient method. For example, the conductance of the leak may beestimated as the instantaneous mask airflow, low-pass filtered with atime constant of 10 seconds, divided by the similarly low-pass filteredsquare root of the instantaneous mask pressure, and the instantaneousleakage flow may then be calculated as the conductance multiplied by thesquare root of the instantaneous mask pressure. Respiratory airflow isthen calculated as the instantaneous mask airflow minus theinstantaneous leakage flow.

In the simple case of no intrinsic PEEP, the instantaneous pressure atthe mask may be simply set as follows, in order to provide ventilatorysupport to the patient:

-   -   P=P_(INSP) flow>0 (inspiration)    -   P=P_(EXP) flow<=0 (expiration)        where P_(EXP) is less than or equal to P_(INSP). Typically,        P_(EXP) might be zero, and P_(INSP) might be of the order of 10        to 20 cmH₂O.

Two embodiments for deriving a measure of the degree of expiratoryairflow limitation will now be considered. In the first embodiment,airway conductance during inspiration is compared with airwayconductance during expiration, and a higher conductance duringinspiration indicates expiratory airflow limitation. Airway conductanceis calculated by superimposing on the instantaneous mask pressure a 4-Hzoscillation of amplitude 1 cmH₂O, and measuring the component of therespiratory airflow signal at 4 Hz. The conductance may be calculatedonce for each half cycle of the 4-Hz oscillation. In order to identifyinspiratory and expiratory halves of the respiratory cycle, therespiratory airflow is low-pass filtered to minimize the imposed 4-Hzoscillation, for example, by averaging measured respiratory airflow overa moving window of length 0.25 seconds. If the 4-Hz low-pass filteredflow is above a threshold such as 0.1 L/sec, it is taken to be theinspiratory half-cycle. Otherwise, it is taken as being the expiratoryhalf-cycle.

Conductance over one or more inspiratory half-cycles, and over one ormore expiratory half cycles is now calculated, using standard averagingor filtering techniques. The conductance during inspiration minus theconductance during expiration yields a first measure M₁ of the degree ofdynamic airway compression. Preferably, M₁ can be normalized by dividingby the mean conductance over the entire breath or breaths, and athreshold value, for example, 0.2, can be subtracted so that onlydifferences in conductance of 20% or more are regarded as indicative ofdynamic airway compression. Thus, M₁=(average conductance duringinspiration−average conductance during expiration)/(average conductanceover entire breath)−0.2.

Finally, it is necessary to adjust the expiratory pressure toservo-control the difference in conductance to be zero. This can be donefor, example, by increasing P_(EXP) by (0.1)(M₁) cmH₂O per second. Usingthis method, if there is dynamic airway compression, P_(EXP) will slowlyincrease until M₁ reaches zero, at which point there will be no furtherdynamic airway compression. Changes in the pressure required to preventdynamic compression with the passage of time can be tracked. In anelaboration of this first embodiment, M₁ can be calculated as a functionof the time into expiration, and the pressure at different points intoexpiration servo-controlled separately within a breath.

In the second embodiment for deriving a measure of the degree ofexpiratory airflow limitation, the degree of expiratory flow limitationis calculated from the shape of the expiratory flow versus time curve.The expiratory portion of each breath is identified, for example, bytaking expiration as the period where airflow is less than 0.1 L/sec.The mean expiratory airflow during the final 25% of expiratory durationis calculated, and divided by the peak expiratory airflow. For a subjectwithout expiratory airflow limitation, this ratio will be close to zero,and less than a threshold such as 0.2, whereas for a subject withexpiratory airflow limitation, it will be larger, for example, in therange 0.2 to 0.6, with higher values indicating more severe dynamicairway compression. Therefore, a second measure of the degree ofexpiratory airflow limitation is M₂=(mean expiratory flow during last25% of expiratory time)/(peak expiratory flow)−threshold, where thethreshold is, for example, 0.2.

In the final step in this second embodiment, if M₂ is positive, theexpiratory pressure P_(EXP) is increased slightly, for example by(0.1)(M₂) cmH₂O per breath. Conversely, if M₂ is negative, P_(EXP) isdecreased slightly, for example, by (0.1)(M₂) cmH₂O per breath.

A third embodiment, which can be used as an enhancement of theservo-controlling step in either of the above two embodiments, takesaccount of the fact that there is no dynamic compression at the start ofexpiration, and no external pressure is required to prevent dynamiccompression at the start of expiration, but that dynamic compressiondevelops as the elastic recoil decreases. Since the elastic recoilpressure decreases approximately linearly on expired volume, theexternal pressure required to be applied will increase approximatelylinearly as a function of expired volume. Therefore, in this thirdembodiment, expiratory pressure is set as:P _(EXP)(t)=KV(t)/V _(T)where P_(EXP)(t) is the pressure at time t in the expiratory portion ofa respiratory cycle, V(t) is the expired volume at time t into theexpiration, and V_(T) is the tidal volume of the previous inspiration.Thus, V(t)/V_(T) increases from 0 to 1 during expiration. The constant Kis adjusted in order to servo-control either M₁ or M₂ to be zero, andwill approximate PEEPi.

Although the invention has been described with reference to particularembodiments, it is to be understood that these embodiments are merelyillustrative of the application of the principles of the invention.Numerous modifications may be made therein and other arrangements may bedevised without departing from the spirit and scope of the invention.

1. A method for preventing dynamic airway compression during ventilatory support of a patient comprising the steps of: providing an interface to the patient's airway, providing said interface with a supply of breathable gas at a controllable pressure and with an exhaust, determining the patient's respiratory airflow, varying the pressure at the patient interface so as to provide ventilatory support to the patient, from at least the respiratory airflow determining the patient's airway conductance separately during the inspiratory and expiratory portions of at least one respiratory cycle and calculating a measure of the degree of dynamic airway compression as a function of the values of the inspiratory conductance and the expiratory conductance, and servo-controlling the measure of the degree of dynamic airway compression by increasing expiratory pressure if said measure is large or increasing, and reducing expiratory pressure if said measure is small or zero, wherein the measure of the degree of dynamic airway compression is calculated as a function of the difference between the inspiratory conductance and the expiratory conductance, or as a function of the ratio of the inspiratory conductance to the expiratory conductance.
 2. The method as in claim 1 in which the separate conductances during inspiration and expiration are determined in accordance with the sub-steps of: superimposing a high-frequency oscillation on the patient interface pressure at a determined amplitude, identifying the inspiratory and expiratory portions of a respiratory cycle, measuring the component of the respiratory airflow at said high frequency separately over the inspiratory and expiratory portions of at least one respiratory cycle, and from the high-frequency component airflow measurements and the determined pressure amplitude, calculating the inspiratory airway conductance and the expiratory airway conductance.
 3. A method for preventing dynamic airway compression during ventilatory support of a patient comprising the steps of: providing a variable pressure to the patient's airway so as to provide ventilatory support, determining the patient's respiratory airflow, measuring the patient's airway conductance separately during the inspiratory and expiratory portions of at least one respiratory cycle and calculating as a function of the determined respiratory airflow and the values of the inspiratory and expiratory conductances a measure of the degree of the patient's dynamic airway compression, and increasing or decreasing expiratory pressure in accordance with the calculated measure of the degree of dynamic airway compression, wherein the measure of the degree of dynamic airway compression is calculated as a function of the ratio of the inspiratory conductance to the expiratory conductance, or from the expiratory airflow versus time curve.
 4. The method as in claim 3 in which, when the measure of the degree of dynamic airway compression is calculated from the expiratory airflow versus time curve, said measure is zero when the expiratory airflow decays exponentially from the moment of the peak expiratory airflow to end expiration, but is large when the expiratory airflow decreases suddenly from the peak expiratory flow and is then steady but non-zero for the remainder of expiration.
 5. An apparatus for determining a pressure response to ameliorate dynamic airway compression during ventilatory support comprising: an interface to a patient's airway and an exhaust, a means for supplying breathable gas at a controllable pressure to the interface, a means for deriving an airflow signal from said interface, and a controller means adapted for: (a) controlling a variation of the pressure at the interface to provide ventilatory support to the patient; (b) controlling a determination of a measure of the degree of dynamic airway compression at least from data representing the airflow signal; and (c) servo-controlling the measure of the degree of dynamic airway compression by increasing the controllable pressure during expiration if the measure of the degree of dynamic airway compression is large or increasing, and decreasing the controllable pressure during expiration if the measure of the degree of dynamic airway compression is small or zero, wherein said measure of the degree of dynamic airway compression is derived by measuring the patient's airway conductance separately during the inspiratory and expiratory portions of at least one respiratory cycle, and calculating the measure of the degree of dynamic airway compression as a function of the values of the inspiratory and expiratory conductances.
 6. The apparatus of claim 5 in which the measure of the degree of dynamic airway compression is calculated as a function of the difference between the inspiratory conductance and the expiratory conductance.
 7. The apparatus of claim 5 in which the measure of the degree of dynamic airway compression is calculated as a function of the ratio of the inspiratory conductance to the expiratory conductance.
 8. The apparatus of claim 5 in which the separate conductances during inspiration and expiration are measured by superimposing a high-frequency oscillation on the patient interface pressure at a determined amplitude, identifying the inspiratory and expiratory portions of a respiratory cycle, measuring the component of the respiratory airflow at said high frequency separately over the inspiratory and expiratory portions of at least one respiratory cycle, and from the high-frequency component airflow measurements and the determined pressure amplitude, calculating the inspiratory airway conductance and the expiratory airway conductance.
 9. An apparatus for determining a pressure response to ameliorate dynamic airway compression during ventilatory support comprising: an interface to a patient's airway and an exhaust, a means for supplying breathable gas at a controllable pressure to the interface, a transducer for determining an airflow signal from said interface, and a processor with programmed instructions for controlling: (a) a variation of the pressure at the interface to provide ventilatory support to the patient; (b) a determination of a measure of a degree of dynamic airway compression as a function of data representing the airflow signal; and (c) increasing or decreasing expiratory pressure in accordance with the derived measure of the degree of dynamic airway compression, wherein said measure of the degree of dynamic airway compression is derived in accordance with instructions controlling measurement of the patient's airway conductance separately during the inspiratory and expiratory portions of at least one respiratory cycle, and calculation of the measure of the degree of dynamic airway compression as a function of the values of the inspiratory conductance and the expiratory conductance.
 10. The apparatus of claim 9 in which the measure of the degree of dynamic airway compression is calculated as a function of the ratio of the inspiratory conductance to the expiratory conductance.
 11. The apparatus of claim 9 in which the measure of the degree of dynamic airway compression is derived from the shape of the expiratory airflow versus time curve.
 12. The apparatus of claim 11 in which said measure is zero when the expiratory airflow decays exponentially from the moment of the peak expiratory airflow to end expiration, but is large when the expiratory airflow decreases suddenly from the peak expiratory flow and is then steady but non-zero for the remainder of expiration.
 13. A storage medium with programmed instructions for use by an electronic processor to control a determination of a pressure response to ameliorate dynamic airway compression during ventilatory support, the instructions controlling the execution of the steps of: (a) varying a pressure value so as to control a provision of ventilatory support; (b) determining a measure of a degree of dynamic airway compression at least from calculated data representing measured airflow; and (c) servo-controlling the measure of the degree of dynamic airway compression by increasing the controllable pressure during expiration if the measure of the degree of dynamic airway compression indicates a need therefor, and decreasing the controllable pressure during expiration if the measure of the degree of dynamic airway compression is small or zero, wherein said measure of the degree of dynamic airway compression is derived in accordance with instructions controlling the sub-steps of: measuring the patient's airway conductance separately during the inspiratory and expiratory portions of at least one respiratory cycle, and calculating the measure of the degree of dynamic airway compression as a function of the values of the inspiratory conductance and the expiratory conductance.
 14. The medium of claim 13 in which the measure of the degree of dynamic airway compression is calculated as a function of the difference between the inspiratory conductance and the expiratory conductance.
 15. The medium of claim 13 in which the measure of the degree of dynamic airway conductance is calculated as a function of the ratio of the inspiratory conductance to the expiratory conductance.
 16. The medium of claim 13 in which the separate conductances during inspiration and expiration are measured in accordance with instructions controlling the sub-steps of: superimposing a high-frequency oscillation on the variable pressure provided to the patient's airway, identifying the inspiratory and expiratory portions of each respiratory cycle, determining a component of the respiratory airflow at said high frequency separately over the inspiratory and expiratory portions of at least one respiratory cycle, and calculating the inspiratory and expiratory airway conductances from the high-frequency component airflow measurements.
 17. The medium of claim 13 in which said instructions, during expiration, control an increase in expiratory pressure approximately linearly as a function of expired volume.
 18. A method for preventing dynamic airway compression during ventilatory support of a patient comprising the steps of: providing a variable pressure device to supply pressure to the patient's airway so as to provide ventilatory support, providing an automatic apparatus for determining respiratory airflow and quantifying therefrom a degree of dynamic airway compression; determining the patient's respiratory airflow, and increasing or decreasing expiratory pressure automatically in accordance with the quantified degree of dynamic airway compression, wherein expiratory pressure is increased if the quantified degree of dynamic airway compression is large or increasing, and expiratory pressure is reduced if the quantified degree of dynamic airway compression is small or zero.
 19. A method as in claim 18 in which the expiratory pressure is increased or decreased by servo-controlling the quantified degree of dynamic airway compression to automatically increase expiratory pressure if the quantified degree of dynamic airway compression is large or increasing, and automatically reduce expiratory pressure if the quantified degree of dynamic airway compression is small or zero.
 20. A method as in claim 18 in which the quantified degree of dynamic airway compression is based upon a respiratory airflow determination made across a plurality of respiratory cycles.
 21. A method as in claim 18 in which the quantified degree of dynamic airway compression is derived as a function of expiratory conductance.
 22. A method as in claim 21 in which the quantified degree of dynamic airway conductance is derived as a further function of inspiratory conductance. 